2 files changed, 20 insertions, 1 deletions
diff --git a/lib/sqlalchemy/dialects/postgresql/base.py b/lib/sqlalchemy/dialects/postgresql/base.py
index bec987c46..9bfde4768 100644
--- a/lib/sqlalchemy/dialects/postgresql/base.py
+++ b/lib/sqlalchemy/dialects/postgresql/base.py
@@ -1429,6 +1429,7 @@ import re
 from uuid import UUID as _python_UUID
 
 from . import array as _array
+from . import dml
 from . import hstore as _hstore
 from . import json as _json
 from . import ranges as _ranges
@@ -2452,6 +2453,24 @@ class PGCompiler(compiler.SQLCompiler):
 
         return target_text
 
+    @util.memoized_property
+    def _is_safe_for_fast_insert_values_helper(self):
+        # don't allow fast executemany if _post_values_clause is
+        # present and is not an OnConflictDoNothing. what this means
+        # concretely is that the
+        # "fast insert executemany helper" won't be used, in other
+        # words we won't convert "executemany()" of many parameter
+        # sets into a single INSERT with many elements in VALUES.
+        # We can't apply that optimization safely if for example the
+        # statement includes a clause like "ON CONFLICT DO UPDATE"
+
+        return self.insert_single_values_expr is not None and (
+            self.statement._post_values_clause is None
+            or isinstance(
+                self.statement._post_values_clause, dml.OnConflictDoNothing
+            )
+        )
+
     def visit_on_conflict_do_nothing(self, on_conflict, **kw):
 
         target_text = self._on_conflict_target(on_conflict, **kw)
diff --git a/lib/sqlalchemy/dialects/postgresql/psycopg2.py b/lib/sqlalchemy/dialects/postgresql/psycopg2.py
index dddce5a62..391368c5f 100644
--- a/lib/sqlalchemy/dialects/postgresql/psycopg2.py
+++ b/lib/sqlalchemy/dialects/postgresql/psycopg2.py
@@ -712,7 +712,7 @@ class PGDialect_psycopg2(_PGDialect_common_psycopg):
             self.executemany_mode & EXECUTEMANY_VALUES
             and context
             and context.isinsert
-            and context.compiled.insert_single_values_expr
+            and context.compiled._is_safe_for_fast_insert_values_helper
         ):
             executemany_values = (
                 "(%s)" % context.compiled.insert_single_values_expr